The feasibility of using continuous-flow tubular reactors (CFTR) as an efficient research tool for polymerization reactions is investigated. This is a continuation of the extensive effort that had been made at Dow in recent years to set up and employ an electro-thermal microreactor (an ohmically-heated CFTR), which resulted in several internal and external publications and a US Patent. The main focus of this work is to investigate the effect of operating conditions and flow composition, mainly the number of existing phases, on the molecular weight of the polymer. A series of polymerization experiments were performed in single-phase (liquid) and two-phase (vapor-liquid) flow regimes. In single-phase polymerization, the ethylene concentration falls continuously along the length of the reactor. This will have a significant effect on the kinetics of polymerization, particularly the molecular weight of the produced polymer. A key advantage of operating in the two-phase region is that an almost constant ethylene concentration is maintained along the length of the reactor. In effect, the vapor phase serves as a reservoir that replenishes the ethylene consumed in the liquid phase by polymerization. The molecular weight data show that this assumption is valid provided that the rate of mass transfer is significantly higher than the rate of the polymerization reaction.